Chapter 4. Advances in Air-Source Heat Pump Water Heaters
Résumé
A heat pump enables one to transfer heat from a heat source to a heat sink using mechanical work. In water heating systems, the heat sink is water and the heat source can be ambient or exhaust air, the ground in a closed water loop, groundwater in an open loop or any other medium in which heat can be recovered. In normal operating conditions, the produced thermal energy is at least two times higher than the mechanical work consumed. That is why heat pumping technology can be interesting compared to conventional boilers or electric heaters. This chapter deals with vapour-compression air-source heat pumps applied to water systems for space heating and domestic hot water production. First, the classic subcritical heat pump cycle is detailed in terms of processes by means of Mollier diagrams. Basic notions will be given concerning the efficiency of the cycle (coefficient of performance, exergetic efficiency) depending on different parameters such as temperature, high and low pressure, subcooling and superheating. This cycle is then compared to the transcritical cycle used with CO2 for hot water production. The list of advantages (such as temperature glide at high pressure) and drawbacks (such as lower efficiency for low temperature water heating) of this cycle will tend to limit the use of the CO2 transcritical cycle to DHW production. Secondly, some of the major improvements brought to the vapour compression cycle will be reported through works made by the research community on multi-stage and inverter compressors, expansion devices, such as ejectors and work recovery devices and the design or the specific use of heat exchangers. Multi-stage compressors enable one to achieve higher pressure ratios and to reduce the heat losses during the compression. Ejectors recover some kinetic energy of expansion and increase the suction pressure at the compressor. Work recovery devices produce some mechanical work that can be reused at compression to reduce the energy consumption. Other improvements are made using internal heat exchangers, desuperheaters for domestic hot water production, microchannels instead of regular tubes for refrigerant and a special fin design to enhance the convection heat transfer coefficient. Heat pumps can also satisfy simultaneous needs in heating and cooling to carry out energy savings. Finally, dealing with air-source heat pumps compels to examine two points: the frosting problems and the defrosting strategies. Frosting reduces the global performance by diminishing the air flow cross section area and by adding a thermal resistance between air and refrigerant. It appears when the ambient air temperature is low and grows faster when the air humidity is high. These conditions occur during winter, when the needs in space heating are high. So an efficient defrosting strategy becomes necessary to ensure the user a satisfactory winter seasonal performance. Conventional defrosting strategies such as hot-gas bypass, reversed-cycle, electric heating and other strategies more in a research phase using air recirculation or a two-phase thermosiphon are discussed in the final part of this chapter.